Sulfate isotope fractionation

R. W. FairbriiXjE, Encyclopedia of Geochemistry and Environmental Sciences, Van Nostrand, New York, 1972.. See sections on Geochemical Classification of the Elements Sulfates Sulfate Reduction-Microbial Sulfides Sulfosalts Sulfur Sulfur Cycle Sulfur Isotope Fractionation in Biological Processes, etc., pp. 1123 - 58. 

Isotope fractionation during sulfate reduction by the hyperthermophilic Archaeoglobus fulgidus varied with the concentration of sulfate, and it was suggested that different pathways were operative at concentrations >0.6 or <0.3 mM (Habicht et al. 2005). 

Habicht KS, L Sailing, B Thamdrup, DE Canfield (2005) Effect of low sulfate concentrations on lactate oxidation and isotope fractionation during sulfate reduction by Archaeoglobus fulgidus strain Z. Appl Environ Microbiol 71 3110-3111. 

Somsamak P, HH Richnow, MM Haggblom (2006) Carbon isotope fractionation during anaerobic degradation of methyl tert-butyl ether under sulfate-reducing and methanogenic conditions. Appl Environ Microbiol 72 1157-1163. 

Shih C-C, ME Davey, J Zhou, JM Tiedje, CS Criddle (1996) Effects of phenol feeding pattern on microbial community structure and cometabolism of trichloroethylene. Appl Environ Microbiol 62 2953-2960. Somsamak P, HH Richnow, MM Haggblom (2005) Carbon isotope fractionation during anaerobic biotransformation of methyl ferf-butyl ether and ferf-amyl methyl ether. Environ Sci Technol 39 103-109. Somsamak P, RM Cowan, MM Haggblom (2001) Anaerobic biotransformation of fuel oxygenates under sulfate-reducing conditions. EEMS Microbiol Ecol 37 259-264. 

Sulfate reduction is dominated by bacterial processes in nature, and several studies have revealed strong variability in the isotopic fractionation (Thode et al. 1951 Harrison and Thode 1958 Kaplan and Rittenberg 1964 Kemp and Thode 1968 Rees 1973 Dickman and Thode 1990 Habicht and Canfield 1997 Brtlchert et al. 2001). The fractionation appears to depend both on the microbial species and on the metabohc state of an individual species (Kaplan and Rittenberg 1964). This is discussed in greater detail below. 

Microbial reduction of nitrate to N2, known as denitrification, is similar. It is kinetically inhibited in the absence of bacteria and is known to induce a kinetic isotope effect (Blackmer and Bremner 1977 Kohl and Shearer 1978 Mariotti et al. 1981 Bryan et al. 1983 Htibner 1986 Mariotti et al. 1988). W N shifts ranging from 6.5%o to 20%o have been observed experimentally. As with sulfate, microbial fractionations appear to depend on the metabolic states of the microbes. 

The body of research on isotopic fractionation induced by sulfate and nitrate reduction provides insight into selenate, selenite and chromate reduction. For sulfate and nitrate oxyanions, reduction is generally microbially mediated, is irreversible, and involves a fairly large but variable isotopic fractionation. As described below, Se and Cr oxyanion reduction follows suit, though abiotic reactions may have a greater role in some transformations. 

If the cell is well supplied with nutrients, then the production of activated enzyme is great and this step is relatively fast. If the transport of sulfate into the cell cannot keep up with the reduction of sulfate, the concentration of sulfate within the cell becomes small, and very little of the isotopically fractionated sulfate inside the cell can leak back out of the cell. Thus, the effect of the internal isotopic fractionation on the outside world is minimal and the overall fractionation of the process is small. In a hypothetical extreme case, every sulfate anion entering the cell would be consumed by reduction. This would require a complete lack of isotopic fractionation, because when all S atoms entering are consumed, there can be no selection of light vs. heavy isotopes. The isotopic fractionation of the overall reduction reaction would be equal to that which occurs during the diffusion step only. 

This may seem paradoxical, as the kinetic isotope effect induced by S-O bond breakage still exists. How can the overall reaction have little isotopic fractionation when one step within it has a large kinetic isotope effect The key to understanding this is in the isotopic composition of the intermediate species in the reaction chain. An intermediate may become enriched in heavier isotopes if the next step in the reaction chain preferentially consumes lighter isotopes. In the hypothetical case described above, at steady state the sulfate within the cell is enriched in the heavy isotope by an amount equal to the kinetic isotope effect occurring at step 2. Thus, the isotopic composition of the flux of S through step 2 is the same as that of the flux of S into the cell and the kinetic isotope effect occurring at step 2 has no effect on the overall isotopic fractionation. 

This lack of a concentration dependence contrasts with the sulfur isotope literature, which suggests that sulfur isotope fractionation induced by sulfate reduction decreases as the sulfate concentration decreases below 0.2 mmol/L (Canfield 2001 Habicht et al. 2002). This difference may reflect differences between S(VI) and Se(VI) reduction pathways or possible adaptations of bacteria to low Se concentrations, but at present no clear explanation has emerged. 

It is possible that a branching reaction could cause isotopic fractionation of Se removed from a Se(0) precipitate, but no evidence exists for this at present. Elemental sulfur can be converted to sulfate and sulfide through a branching reaction mediated by disproportionating bacteria, and the produced sulfate s ratio is shifted +17%o to +31%o relative to the... 

Bruchert V, Knoblauch C, Jorgensen BB (2001) Controls on stable sulfur isotope fractionation during bacterial sulfate reduction in Arctic sediments. Geochim Cosmochim Acta 65 763-776 Bryan BA, Shearer G, Skeeters JL, Kohl DH (1983) Variable expression of the nitrogen isotope effect associated with denitrification of nitrate. J Biol Chem 258 8613-8617 Canfield DE (2001) Biogeochemistry of sulfur isotopes. Rev Mineral Geochem 43 607-636 Chau YK, Riley JP (1965) The determination of selenium in sea water, silicates, and marine organisms. Anal Chim Acta 33 36-49... 

Habicht KS, Canfield DE (1997) Sulfur isotope fractionation during bacterial sulfate reduction in organic-rich sediments. Geochim CosmochimActa61(24) 5351-5361 Habicht KS, Gade M, Thamdrup B, Berg P, Canfield DE (2002) Calibration of sulfate levels in the Archean ocean. Science 298 2372-2374... 

Johnson TM, Bullen TD (2003) Selenium isotope fractionation during reduction by Fe(ll)-Fe(lll) hydroxide-sulfate (green rust). Geochim Cosmochim Acta 67 413-419 Johnson TM, Bullen TD, Zawislanski PT (2000) Selenium stable isotope ratios as indicators of sources and cycling of selenium Results from the northern reach of San Francisco Bay. Environ Sci Tech 34 ... 

Thode HG, Kleerekoper H, McElcheran DE (1951) Isotope fractionation in the bacterial reduction of sulfate. Research Lond 4 581... 

A clear avenue of future research is to explore the S-Fe redox couple in biologic systems. Bacterial sulfate reduction and DIR may be spatially decoupled, dependent upon the distribution of poorly crystalline ferric hydroxides and sulfate (e.g., Canfield et al. 1993 Thamdrup and Canfield 1996), or may be closely associated in low-suUate environments. Production of FIjS from bacterial sulfate reduction may quickly react with Fefll) to form iron sulfides (e.g., Sorensen and Jeorgensen 1987 Thamdrup et al. 1994). In addition to these reactions, Fe(III) reduchon may be coupled to oxidation of reduced S (e.g., Thamdrup and Canfield 1996), where the net result is that S and Fe may be cycled extensively before they find themselves in the inventory of sedimentary rocks (e.g., Canfield et al. 1993). Investigation of both S and Fe isotope fractionations produced during biochemical cycling of these elements will be an important future avenue of research that will bear on our understanding of the isotopic variations of these elements in both modem and ancient environments. 

In marine coastal sediments typically 90% of the sulfide produced during snlfate reduction is reoxidized (Canfield and Teske 1996). The pathways of snlfide oxidation are poorly known bnt inclnde oxidation to sulfate, elemental snlfm and other intermediate componnds. Systematic studies of sulfm isotope fractionations dnring sulfide oxidation are still needed, the few available data snggest that biologically mediated oxidation of snlfide to elemental snlfur and sulfate lead to only minimal isotope fractionation. 

However, in contrast to microbiological experiments and near-surface studies, modelling of sulfate reduction in pore water profiles with in the ODP program has demonstrated that natural populations are able to fractionate S-isotopes by up to more than 70%c (Wortmann et al. 2001 Rudnicki et al. 2001). Brunner et al. (2005) suggested that S isotope fractionations of around -70%c might occur under hyper-sulfidic, substrate limited, but nonlimited supply of sulfate, conditions without the need of alternate pathways involving the oxidative sulfur cycle. 

Fig. 2.21 Rayfeigh plot for sulphur isotopic fractionations during the reduction of sulfate in a closed system. Assumed fractionation factor. 1.025, assumed starting composition of initial sulfate ...

Finally it should be mentioned that sulfate is labeled with two biogeochemical isotope systems, sulfur and oxygen. Coupled isotope fractionations of both sulfur and oxygen isotopes have been observed in experiments (Mizutani and Rafter 1973 Fritz et al. 1989 Bottcher et al. 2001) and in natnrally occurring sediments (Ku et al. 1999 Aharon and Fn 2000 Wortmann et al. 2001). Brunner et al. (2005) argned that characteristic fractionation slopes do not exist, but depend... 

An increase in oxygen fugacities has a much stronger effect on the 5 " S-values than a pH change, because of the large isotope fractionation between sulfate and sulfide. Figure 3.13 shows an example of the effect of pH and f02 variation on the... 

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